Manufacture of continuous material webs of fibrous particles at high consistencies by passing particles through a series of constrictions

ABSTRACT

Sheets of fibrous particles such as paper sheets are formed from high consistency suspensions of fibrous particles. The consistency of the fibrous particles in the suspension is not less than twice the sediment concentration, i.e., 1-6 percent by weight and preferably 4-5 percent by weight, the fibres having lengths of 1-5 mm or longer. The fibres are first dispersed by forcing the fibre suspension through a series of constrictions and deflected after each such constriction to produce a turbulent flow insuring uniform fibre distribution and the turbulent flow is then decayed to cause the fibres to consolidate as a three dimensional network structure with entrained liquid. The network is then deposited on a paper machine wire or felt. Apparatus for the high consistency formation process is described.

United States Patent Wahren et al.

[ MANUFACTURE OF CONTINUOUS MATERIAL WEBS OF FIBROUS PARTICLES AT HIGHCONSISTENCIES BY PASSING PARTICLES THROUGH A SERIES OF CONSTRICTIONS[75] Inventors: Douglas Wahren, Taby; Lennart Reiner, Gustavsberg, bothof Sweden [73] Assignee: A. Ahlstrom Osakeyhtio,

Noorrnarkku, Finland [22] Filed: Dec. 20, 1973 [21] Appl. No.: 427,024

Related U.S. Application Data [63] Continuation-impart of Ser. No.157,120, June 28,

1971, abandoned.

[52] U.S. Cl. 162/216; 162/293; 162/336;

[51] Int. Cl. D2lf l/06 [58] Field of Search 162/216, 336, 338, 343,

[56] References Cited UNITED STATES PATENTS 2,205,693 6/1940 Milnel62/343 June 3, 1975 3,216,892 11/1965 Wahlstrbm 162/343 3,661,7025/1972 Means 162/216 3,769,155 10/1973 Schiel 162/343 Primary ExaminerS.Leon Bashore Assistant Examiner-Richard V. Fisher Attorney, Agent, orFirmBrooks, Haidt, Haffner & Delahunty [5 7] ABSTRACT Sheets of fibrousparticles such as paper sheets are formed from high consistencysuspensions of fibrous particles. The consistency of the fibrousparticles in the suspension is not less than twice the sedimentconcentration, i.e., 1-6 percent by weight and preferably 4-5 percent byweight, the fibres having lengths of l-5 mm or longer. The fibres arefirst dispersed by forcing the fibre suspension through a series ofconstrictions and deflected after each such constriction to produce aturbulent flow insuring uniform fibre distribution and the turbulentflow is then decayed to cause the fibres to consolidate as a threedimensional network structure with entrained liquid. The network is thendeposited on a paper machine wire or felt. Apparatus for the highconsistency formation process is described.

13 Claims, 10 Drawing Figures SHEET PATENTEU 3 I975 fiwm l I 3 /////4 r4 I 7///// THEMED JUH 3 I975 SHEET MANUFACTURE OF CONTINUOUS MATERIALWEBS OF FIBROUS PARTICLES AT HIGH CONSISTENCIES BY PASSING PARTICLESTHROUGH A SERIES OF CONSTRICTIONS CROSS REFERENCE TO RELATED APPLICATIONThis application is a continuation-in-part of applicants prior copendingapplication, Ser. No. 157,120 filed June 28, 1971, entitled MANUFACTUREOF CONTINUOUS MATERIAL WEBS OF FIBROUS PARTICLES, and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to the making of continuous material webs from fibre suspensionsand particularly to a process and apparatus at the wet end of a webforming machine.

2. Description of the Prior Art The method of manufacturing paper on anindustrial scale has been substantially the same since the beginning ofthe 19th century. The only changes made relate to machine dimensions andweb speed, which both have been increased. Owing to the greater machinewidths and speeds, the machine parts have to be manufactured with anincreasingly higher degree of precision. This again has resulted inconsiderable cost increases. As an example it is estimated that theinvest ment cost for a modern newsprint machine with peripheralequipment and buildings comes close to 150 million Swedish Crowns. Alarge part of the machine investment cost is required for its wet part,i.e., the distribution system for the suspension, the headbox and wiresection.

At the wet end of a paper machine, the procedures for conventional sheetformation are basically as follows: A fibre suspension of more or lessfreely moving cellulose fibres in water is distributed coarsely acrossthe width of the machine by a distribution system, such as across-machine distributor. The headbox of the machine is also intendedto uniformly distribute the fibres on a small scale by means ofirregular movements (turbulence) of the transporting medium. In order toeliminate certain deficiencies in the distribution system (involving,for example, an oblique velocity profile in the headbox which inaddition to a non-uniform velocity profile across the paper web alsoindirectly gives rise to an unstable flow with a large scale turbulence,which becomes apparent in the wire section and disturbs the sheetformation), in most cases a number (two to five) perforated rolls areplaced in the flow path. The fibres in the suspension, formechanical-geometrical reasons, have a tendency to fiocculate. A furtherobject of the perforated rolls is to produce turbulent shear fields forbreaking up the fibre flocks developed. Owing to the tendency of thefibres to form flocs, particularly at increased concentrations, thefibre concentration in conventional processes must not exceed about 0.5percent if acceptable paper is to be produced. By 0.5 percentconcentration is meant a concentration of g of fibres per kg of water.

From the headbox the suspension, optimally with uniformly distributedfibres, is made to flow out through a narrow gap Slice as a nearlyhorizontal jet on to the wire (a more or less open metal or plastic wirecloth) advancing with approximately the same speed as the web. Thethickness of the jet can vary from a few mm up to and above 50 mm. Onthe wire, most of the water is to be removed. Prior to the fixing of thefibres in a fibre bed, the concentration is to be increased from 0.5percent to about 10 percent. At a jet height of 40 mm and with anoriginal fibre concentration of 0.5 percent, thus, about 40 litres ofwater per square meter of the wire have to be removed. At the machinespeeds now employed in high-speed machines, this water removal has totake place within a period of about 1 second. The water is removed bymeans of different types of dewatering units which according to thecircumstances may improve or worsen the sheet formation. In any casethis conventional process is very difficult to control.

At the moment of contact between the jet and the wire, the wire hasapproximately the same speed as the jet. The sheet formation proper,therefore, can be compared to a filtration process with sedimentationforced by the dewatering units. The sheet is built up from below in sucha way that the last water remaining must be drained throughsubstantially the entire sheet. The fibres included in the sheet have acertain size distribution always containing a greater or smaller amountof fines, i.e., fibre fragments, that are so small that they followalong with the water being drained. The retention, that is, the fractionof the fibrous material remaining on the wire, often amounts to only 50percent or even less. This kind of water removal process imparts acertain two-sidedness to the sheet. In other words, the bottom or wireside of the sheet is lacking in fines content, while at the same timethere is an excess of such material toward the upper surface of thesheet. This two-sidedness is particularly distinct when fillers such asclay are added to the suspension as is the case with certain grades ofprinting paper. This two-sidedness is also a characteristic of someligno-cellulose-containing papers, for example, newsprint, where a highpercentage of fines is contained in the furnish. This gives the twosurfaces of the sheet different printing properties. Owing to theaforesaid analogy between the mechanism of sheet formation andfiltration with sedimentation, the sheet is formed with a specialtwo-dimensional structure. All fibres, because of their geometric shape(length l-5 mm, diameter 30-50 pm), are oriented in such a way thattheir main extension is essentially parallel to the plane of the sheet.The sheet, thus, can be said to be built up of a number of essentiallyparallel layers. This structure, of course, affects such paperproperties as strength, stiffness, etc.

The above simplified description basically sets forth the usual processof sheet formation in present paper manufacturing technology. Otherprocesses do exist, but they do not differ very much in principle fromwhat has been described. The cellulose fibres are deposited on a wirecloth or between two wire cloths in one way or another. When thedewatering operation has proceeded far enough that the strength of theformed sheet allows it, the sheet is transferred to the press sectionfor a further dewatering. The final dry content of the paper is obtainedafter drying of the paper against a number of heated cylinders.

In the Finnish Pat. No. 40,263, hereby incorporated by reference, thereis disclosed a paper machine head box wherein a suspension flow isdistributed by allowing it to pass through a fixed apertured plateextending crosswise in relation to the paper machine. Downstream inclose proximity to this apertured plate there is a second aperturedplate having the same number of apertures as the first mentioned platebut staggered with respect thereto, whereby the total cross-sectionalarea of the apertures in the second plate is greater than that of thefirst mentioned plate.

Such apparatus operates only with suspensions having a low concentrationwherein the tendency of flocculation is relatively low and it should benoted that the fibres are loose when discharged from this head box as asuspension flow.

An attempt to secure uniform distribution of flow across the width of aweb is illustrated by Milnes British patent specification No. 469,203 of1937, relating to a headbox for conventional fibre concentrations.According to the Milne specification a suspension is allowed to flowthrough a series of baffles, which would incidentally generate lowintensity turbulence, but there is no suggestion of generating intenseturbulence by passing a suspension through very small openings andMilnes drawing shows large passages. Nor is flow consolidated afterpassage through Milnes headbox.

US Pat. No. 3,652,392 to Appel represents an attempt to distribute asuspension evenly without using distributor rolls. Appel sought tocontrol turbulence through his inlet arrangement so that small scaleturbulence would be preserved as the suspension moves on to a wire. Asin other prior art headbox and inlet arrangements this Appel devicerelied on the belief that it is necessary to avoid excessiveflocculation to form a good sheet from a very low fibre concentrationsuspension.

U.S. Pat. Nos. 3,514,372 and 3,216,892 relate to headbox arrangementswherein suspensions pass through passages and apertures of successivelyincreasing areas.

These and other prior art attempts to improve web formation usingconventional fibre concentrations have proposed various ways ofcontrolling either flow distribution or turbulence or both, alwaysbasing their methods and devices on the idea that a very low fibreconcentration must be employed in order to avoid excessive flocculationin the suspension and poor sheet formation.

The disadvantages of prior art techniques are eliminated by the methodand apparatus of the present invention, which employ unconventionallyhigh fibre concentrations and, instead of avoiding flocculation, put thetendency of such high concentrations to form flocs to use to form auniform sheet.

The present invention relates to a novel method wherein the web isconsolidated in the head box to a three-dimensional network structurewith entrained liquid.

SUMMARY OF THE INVENTION Fibres suspended in water have, as mentionedabove, the tendency of agglomerating and flocculating into localnetworks. This tendency is due to several factors. A factor of decisiveinfluence is the high length-toradius ratio of the fibres, combined withthe fact that the fibres have a certain stiffness under certaincircumstances. It is then possible for the fibres to become fixedmechanically relative to one another and to remain in these fixedpositions. This circumstance, which has caused much trouble inconventional paper manufacturing processes, is utilized in the presentinvention for the sheet formation. Instead of trying to preventflocculation, flocculation is facilitated to the highest possible degreeand in such a way that a continuous network develops. To develop such anetwork requires a sufficiently great number of fibres per unit ofvolume, i.e., the concentration must be high. To obtain a continuousnetwork, all local networks with a high density must be dispersed so asto render it possible for the individual fibres to assume new locationsin the continuous network. This implies that relatively high shearingforces must be generated in order to form the new network. Once the newnetwork has been formed, i.e., when the fibres have occupied theirlocations, all disrupting forces have to cease as quickly as possible.It is this new network, that in a more compressed state is to form thesheet of paper. The thickness of the network depends on the desiredbasis weight of the paper to be made and on the fibre concentrationused. With a basis weight of, for instance, 50 g/m and a fibreconcentration of 5 percent, the thickness will be 1 mm.

Shear forces occur in a turbulent flow. Turbulence generally is ameasure of the intensity of random velocity, the function variationsoccurring in a flow under certain circumstances. In turbulent flow,eddies with a certain distribution of sizes occur simultaneously in theflow field. The eddy size distribution largely depends on the geometryof the flow conduit. If the flow space is restricted the size of eddiesoccurring there will also be limited. The intensity of the eddies isdirectly proportional to the power per unit volume of the turbulenceproduced by the creating section of the flow, i.e., in many casesagainst the pressure loss within the section. The rate of damping of theturbulence depends primarily on the size of the eddies and of theviscosity and of possible strength of a fibre network in the flow (theflow energy is transformed to heat via viscous dissipation).

The term sediment concentration, abbreviated C can be defined as thefinal concentration in the sediment formed when fibres are allowed tosettle out from a very dilute suspension. This concentration correspondsto that at which the fibres, after agitation, start to form coherentnetworks. Such network formation causes the formation of flocs whichdisturb sheet formation according to prior art techniques of papermanufacture. Accordingly paper has traditionally been formed fromsuspensions at consistencies ranging from approximately 0.1 to 1 percentby weight, although the consistencies can be higher when the major partof the solids content is made up of filler materials. Paper has normallybeen formed from suspensions wherein the concentration of fibres isbelow, or slightly above the sediment concentration of the fibres.Throughout this application and the appended claims the term sedimentconcentration shall be defined as follows:

where C, is the sediment concentration, r is the fibre radius and L isthe fibre length. Thus C, for pine sulphate suspensions is 0.2 to 0.4percent and for groundwood, 0.6 to 0.9 percent. In contrast to prior artprocesses operating with concentrations of about C the process andapparatus of the present invention employ suspensions havingconcentrations not less than twice the sediment concentration of thesuspended fibres as defined. This minimum concentration of suspensionsformed into webs in accordance with the present invention i.e., twicethe sediment concentration is called the double sediment concentrationThus the present invention deals with concentration ranges of l.0 to 6percent, and preferably 4 to 5 percent which is about times theconcentration normally used in the art.

The length of fibres in the suspension is preferably in the range offrom about l mm to about 5 mm and the width of the fibres is preferablyin the range of from about 30 am to about 50 ,urn. The kinds of fibresused may vary within very wide limits from natural fibres to syntheticfibres and the like. The range of fibre lengths in suspensions treatedin accordance with the invention is such that it is convenient todiscuss fibre length in terms of the average length, i.e., the meanfibrous particle length.

Because of the use of suspensions having concentrations much higher thanthose of the prior art, considerably less water has to be removed fromthe web. Accordingly the loss of fines with the water removed is alsoreduced.

Compared with sheets manufactured by prior methods and arrangements thethree-dimentional sheet manufactured according to the invention providesimproved strength properties in a direction perpendicular to the planeof the sheet. The size of the apparatus according to the invention isdrastically reduced compared with prior constructions and hence theexpenses are remarkably reduced.

It is apparent that the intensity and scale of the turbulent flowrequired to dissipate all fibre flocs depends on the concentration ofthe suspension but the injection rate required for producingconsolidated network structure is readily determinable by an operatorcontrolling the process.

Following the apertures in which the intensity of the turbulent flow isincreased and the scale decreased there is a chamber in which the flowis allowed to decay in order to form the consolidated three-dimensionalnetwork structure. This network structure is then discharged through anoutlet channel which has walls diverging outwardly toward the outlet ofthe head box so as to prevent friction between the channel walls and theformed web from destroying the three-dimensional network structure ofthe web. In effect the web is formed before deposition on the wire. Thepurpose of this diverging inside channel is thus entirely different fromthat of similar channels in prior head boxes in which the fibres arestill separate and not consolidated into a network as in the presentinvention.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows the forming part for the web ina sectional view from the side.

FIG. 2 is a view in section taken along the lines 11-11 of FIG. 1.

FIGS. 1a and 2a are views similar to FIGS. 1 and 2, showing anotherembodiment of the forming part for the web in a sectional view from theside and along the line Ila-Ila in FIG. la; looking in the direction ofthe arrows.

FIGS. 3 and 4 respectively show a distribution unit in a sectional viewfrom the side and along the line IV-IV in FIG. 3.

FIG. 5 shows in a schematic way the entire sheet forming arrangementwith the forming part and groups of distribution units assembled.

FIG. 6 shows an elevational view of a preferred embodiment of theinvention, in which the distributing and web forming parts have beencombined to a single compact unit. 7

FIG. 7 shows a section taken along line c c in FIG. 6.

FIG. 8 shows two sections taken along lines aa and b-b in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the sheet forming methodaccording to the invention the fibre suspension is injected throughclosely spaced nozzles 1, see FIGS. 1 and 2, into a narrow channel 2having a front end portion or chamber 3 the sheet forming zone of aspecial conformation, as appears from FIG. 1, for bringing about certaindesired effects. The object to be achieved here is a turbulent flow witha certain eddy size distribution. The intensity of the eddies depends onthe injection rate of the fibre suspension. The eddy size and intensityare to be balanced so that the turbulence has decayed substantially bythe time the fibre suspension passes the narrow gap 4 constituting thebeginning of the channel 2 proper.

The sheet forming process may be described in greater detail as follows.On injection, a large part of the kinetic energy of the fibre suspensionis converted to turbulent energy appearing in the form of small eddies.Owing to the small size and high intensity of these eddies, everyindividual fibre is affected, thereby rendering it possible for thefibre to arrange itself into a network, the strength of which soonexceeds the shear forces of the decaying turbulent field. During theinitial phase of the forming process all of the material flows in such away that the fibres are caused to selectively arrange themselves into anetwork that is as homogeneous as possible. This condition obtains athigh fibre concentrations. At a concentration of 5 percent there arefrom 100,000 to 500,000 fibres per cubic centimeter of suspension.

Having thus been formed, the network must not be subjected to anydisrupting forces exceeding its strength. The network is compressed atthe passage through the narrow section 4 and, owing to the stressesproduced there and inherent in the forming process, the network willexpand in the course of time. During passage of the network through thechannel 2, the fibres may gradually start to press against the channelwalls. The boundary layers often observed in the plug flow regime andwhich are more or less free of fibres at low concentrations, may not becapable of withstanding the mechanical pressure of the expandingnetwork. If no special measures were taken, the frictional forcesbetween the fibres and the walls of the channel could reach such amagnitude that the network would be partially disrupted. By giving thechannel 2 a somewhat diverging conformation in the flow direction, thereis some space available for expansion of the network. Owing to thedecreasing flow rate in the widening channel, a further positive effectis obtained in that the network is compressed in the direction ofmotion. This appears to have a balancing effect on the microstructure ofthe final sheet. Due to the mechanical strength of the network, the flowdoes not become unstable. On flowing out of the channel the fibrenetwork may be placed on the wire at an arbitrary angle to thehorizontal plane according to conventional sheet forming techniques, butbecause of the high dry content of the network, which can be considereda web or sheet even at this stage, it is also possible in some cases todirect the jet right on to a press felt, between two felts or some suchother arrangement for handling a web, for further transport of the webinto a press or some other means for further dewatering.

FIGS. la and 2a show another embodiment of the forming part. By breakingdown the fibre jet at least one more time after passage through thenozzles 1 and chamber 3, but before the entry of the fibres into thechannel 2, an improved uniformity of the final product can be obtained.In connection with said latter breaking down step, it has provedexpedient also to let the suspension change its direction of motionbefore, at or after it has entered the channel 2.

As in FIGS. 1 and 2, the designation 1 in FIGS. 1a and 2a refers toclosely spaced nozzles feeding into the chamber 3 extending along therow of nozzles. Instead of providing a channel extending in the maindirection of motion of the suspension out of the chamber 3, i.e.,substantially perpendicularly to the direction of motion in the nozzles,the channel 2 according to FIGS. 1a and 2a is arranged so as to extendfrom the chamber 3 at an angle of substantially 90 to the generaldirection of motion which the suspension has when leaving the chamber 3.Between the chamber 3 and the channel 2 an enlargement a extending alongthe width of the channel 2 is arranged as shown in FIGS. la and 2a. Theconnections between the chamber 3 and the space 5a, and between saidspace 5a and the channel 2 are narrow sections 4 and 6a respectively.

In FIGS. la and 2a the channel 2 is shown to extend substantially inparallel with the direction of motion of the suspension in the nozzles,i.e., about 90 to the general direction of motion of the suspension inthe narrow section 4. This provides a compact and, accordingly,advantageous embodiment of the forming device. The channel 2, however,may also be given another suitable direction in relation to the flow inthe narrow section 4. It is possible as well, depending on the resultdesired with respect to the final product, to arrange additionalenlargements 5a in series upstream of the channel 2.

The distribution across the machine of a suspension with a high fibreconcentration poses a special problem. It is not suitable to utilize aconventional distribution system such as a tapered cross-machinedistributor. The highly unstable flow conditions that prevail when thenetwork strength largely controls the flow, as is the case when theconcentration of elongated particles is high, renderes it difficult toobtain a uniform distribution. A distribution unit solving theseproblems is shown in FIGS. 3 and 4. The main flow is accelerated througha converging section 5 in a pipe 6 and, at a high speed, meets a wall 7that forces the flow to assume a radial direction. Radial outlets 8,through which the suspension flows, are arranged in the wall 7. Thefunction of this structure is, as in the forming zone 1, 3, to producehighly turbulent shear fields in order to make the material flow, sothat the fibres can follow the pressure gradient as well as possible.The reference numeral 9 designates an optional inlet for fresh waterthat may be used when starting up.

The number of outlets 8 as well as the necessary distance between themdepends primarily on the size and shape of the fibres. The fibre lengthaffects the choice of the distance between the outlets inasmuch as thefibres should not be able to bridge the distance between two holes,thereby initiating clogging of the outlets 8. The clogging action,however, is not entirely due to one factor, but depends also on theintensity of the turbulent shear fields at the outlets. The dampingprocess of the turbulence, thus, will determine the distance between thecenter and circumference of the pipe. Thus, if the distance between theoutlets 8 and the radius of the pipe 6 are given, the number of outletsis also determined. The damping process can be affected by varying theflow space as mentioned before, but also here it is desirable to have acertain scale of the turbulence (eddy size distribution).

The distribution system for a large machine can be as shown in FIG. 5.In the Figure the sheet forming section is indicated uppermost by thereference numeral 10, and the distribution units shown individually inFIGS. 3 and 4 are arranged in groups indicated as group 1, group 2 andgroup 3. The operating conditions for the distributing units in groups 1and 2 are not as critical as those for the final group 3. The outletdiameters are relatively large and, consequently, no tendency to clogwill occur. However, the scale of the turbulence should be small topromote a uniform fibre and flow distribution even if it is not nearlyas small here as in the forming section 10. The main flow enters group 1comprising one single distribution unit with a number n, of outlets, isthere distributed to group 2 (comprising n units each with n outlets)and from group 2 to group 3 (comprising 11 units each with n outlets),and so on for further distribution until the flow reaches the formingpart 10. The number of distributing units are dependent, as mentionedbefore, on the appearance and concentration of the fibres.

For the transport of the suspension between the different groups 1-3 andfinally to the forming zone 10, pieces of plastic tubing, preferably allof the same length, may be used. The important factors are that thepressure drops along the connections should be equal and an energyabsorption capacity should be embodied to eliminate most of the pressurepulses that may emanate from previous components, such as screens,pumps, etc. 1 Some pertinent points concerning sheet formation at highconcentration according to the method described above may be stated asfollows. The sheet forming unit proper will be very small and requireslittle space. Its manufacture will be simple even at machine widths ofabout 10 m. The choice of material is not a critical factor, because ofthe small size of the surfaces subjected to pressure, which implies arelatively small total load. Plastics, such as, for instance,plexiglass, can be suitable. In addition to certain advantages withrespect to the final product the paper probably the greatest advantageis that the method offers the possibility of radically reducing or evenentirely abandoning the wire section. Since web or sheet structure isalready formed, and only a small fraction of the usual water content isstill present, there is nothing to prevent feeding the sheet directlyinto a press. By abandoning the headbox and wire section, the length ofthe paper machine is reduced by almost 25 percent. The cost of a papermachine is reduced even more, since the headbox and wire sections areabove average in cost. The paper will have properties superior incertain respects to those of conventional paper.

One of the advantageous paper properties obtained in a sheet formedaccording to the invention is the existence of a more three-dimensionalstructure, evolved as the fibres assume their locations in the networkby chance and, thus, will not necessarily be located in the plane of theweb. When the network is compressed in the presses and thereaftersubjected to drying, the chemical bonding strength in the paper will beassisted by a purely mechanical strength, because of the entanglement ofthe fibres. A strength property which is undoubtedly improved is thez-strength, i.e. the strength perpendicular to the plane of the sheet.Furthermore, an improvement of the porosity and bulk values can beexpected, which also may be advantageous for certain grades of paper. Inthe introduction poor retention of fines in conventional formation ofsome grades of sheet was mentioned. In the sheet forming methodaccording to the invention only a relatively small quantity of water hasto be removed. Thus, the flow through the sheet is small and,consequently, the washing out of fines is much reduced. Thereby theextent of material transport will be reduced, which will result inimproved conditions both with respect to two-sidedness and from anenvironmental point of view, because less fines will be discharged withthe waste water. The sheet forming method according to the inventionfurther offers possibilities of better control of the sheet formationfor obtaining 21 better basis weight distribution than is possible withconventional methods.

An especially preferred embodiment of apparatus for practicing theinvention is depicted in FIGS. 6-8, in which a distribution part 20 anda web forming part 21 are combined in a single compound unit head boxhaving an inlet at one side thereof and an outlet slice 23 at the rearend thereof.

The distributing part 20 includes an elongated tubular duct 19 extendingover the entire width of the head box and provided with a side havingconstrictions or outlets in which each outlet is offset with respect toeach adjacent outlet in relation to a horizontal middle plane. Eachoutlet 15 opens into a co-axial circular disclike cavity 16 having awall 17 opposite the outlet 15 and positioned at regular intervals onthe periphery of the cavity 16. The apertures 18 connect thedistributing part 20 with the web forming part 21 which includes acommon chamber 13 divided into a lower and an upper section by thebaffle 14 extending horizontally from a wall opposite the inlet to theoutlet channel 12.

It is seen from FIG. 8 that the apertures 18 are arranged in twohorizontal superposed rows at regular intervals from each other.

The diameter of the outlets 15 is in this special case v"? times thediameter of the apertures 18, i.e., the cross-sectional area of theoutlets 15 is equal to the total cross-sectional area of the aperture18. The diameter of the apertures 18 is at most three times the meanfibrous particle length or smaller.

The aim of the forming operation is to obtain as good a dispersion ofthe fibres as possible and then to let this dispersion be transformedinto a continuous fibre network. The most important factor thatdetermines the degree of dispersion that can be attained is thedispersing power (turbulent energy) per unit volume. This is analogousto the power dissipated by, for instance, a stirrer operated in thesuspension.

The power dissipated per unit volume is determined by the pressure dropacross and average retention time in the forming zone. This can beunderstood in the following way: suppose the fiow rate is q m /s, thevolume of the forming is Vm and the pressure drop across this volume isp N/m". The power dissipated in the forming zone is equal to the powernecessary to force the suspension through the zone. This power is pq(N/m") (m /s) (Nm/s) watt. During the time t the total energy expendedis pqt. This energy is dissipated into the volume of suspension qt.Hence the average energy dis sipated per unit volume of suspension is p(=Nm/m (=ws/m It has been found, however, that this energy must beexpended at a certain rate, i.e., that the power (not the energy) perunit volume is the determining factor. By similar reasoning one thenobtains:

Power per unit volume P/t pq/ V.

Thus it is equivalent if the volume of the forming zone, V, is madesmall, the pressure drop, p, or the flow rate q is made high. Thisleaves considerable freedom in the design of forming zones for differentpurposes.

If only a minimum of dispersion is needed, as, for instance, in aforming zone for a pulp drying machine, a pressure drop, P, in the range/2 1 atm has been found adequate in connection with a total power perunit volume of 2 to 6 X 10 w/m If on the other hand very good dispersionis desired, as in a former for sack paper, the dissipated power per unitvolume has been found to have to be in the range 5 to 15 X 10 w/m.

The pressure drop in the forming zone can be varied without adverseeffects on the sheet characteristics. A good operating range has beenfound to be 3 to 12 X lO Nm As has been pointed out above, however, thedimensioning of the apparatus can be varied considerably as long as theabove parameters are kept within the desired ranges.

As already mentioned, the invention has been described here only by wayof one example. The distribution units, for example, may be designed ina different way, they may be directly assembled with the forming zone,and so on, and the nozzles to the forming zone may be arranged inseveral rows alongside each other. The channel 2, furthermore, may havedifferent lengths, depending on the desired effect, and the nozzles 1may have different dimentions.

Numerous modifications, substitutions and variations of the method andapparatus of the invention will suggest themselves to those skilled inthe art, within the spirit and scope of the invention. What is disclosedis a method and apparatus for high consistency formation of webs offibrous particles by effecting a high pressure drop across a very smallvolume and then causing a consolidated fibre network to form.

What is claimed is:

1. An improved method of manufacturing a continuous material web ofelongated fibrous particles including preparing a suspension of theparticles in a liquid with the concentration of the fibrous particles insuspension being about 1.5 percent to 6.0 percent of the total weight ofthe liquid plus the fibrous particles; then forcing the suspensionthrough a series of constrictions and deflecting the flow of thesuspension after each constriction to create a turbulent flow ensuringuniform distribution over the web; and finally decaying the turbulentflow before deposition of the material and comprising the steps of:

gradually decreasing the scale and increasing the intensity of theturbulent flow in the series of constrictions by distributing the flowfrom one constriction over a plurality of subsequent constrictions ofsmaller diameter and said one constriction, the smallest constrictionsof said series having a diameter not exceeding three times a meanparticle length of said fibrous particles; and

eliminating all disrupting forces after the flow has passed through thesmallest constrictions to transform the turbulent flow into a flow of aconsolidated three-dimensional network structure of fibrous particleswith entrained liquid before deposition thereof.

2. The improved method of claim 1, wherein the flow rate of the networkstructure is successively decreased to the speed of the deposited web.

3. The improved method of claim 1, comprising preparing the suspensionwith a fibre concentration of 4-5 percent by weight.

4. The improved method of claim 1 wherein said smallest constrictionshave a diameter not exceeding about 15 mm.

5. An improved apparatus for the manufacture of a continuous materialweb of fibrous particles having a length of about 1 to 5 mm and of thetype having a base; a support on the base; a headbox attached to thesupport and comprising a transversal apertured front wall and an insidechamber for decaying a turbulent flow before deposition thereof; meansconnected to the headbox for distributing a suspension of the particlesin a liquid over the entire width of the headbox; means connected to thedistributing means for preparing the suspension; and means fortransporting the suspension to the distributing means, the improvementcomprising: inside the headbox at least one other aperture wall in closeproximity to the apertured front wall providing points of impingementfor the turbulent flow forced through apertures to deflect the flow, theapertures in said other wall being smaller than and displaced withrespect to the apertures of said front wall so that each point ofimpingement is surrounded by a plurality of apertures of less diameterthan the next preceding aperture so as to successively decrease thescale and increase the intensity of the turbulent flow and the aperturesof the last of said walls having a diameter not exceeding the threefoldmean particle length; and baffles behind the last apertured walldefining a chamber wherein the turbulent flow is deflected before it ispassed into an outlet channel.

6. The improved apparatus of claim 5, wherein the total area of theapertures in the front wall is equal to the total area of the smallerdiameter apertures in said other apertured wall.

7. The improved apparatus of claim 5, in which the cross-sectional areaof the outlet channel is increased in the flow direction to decrease thevelocity of the network structure flow to the speed of the depositedweb.

8. The improved apparatus of claim 5, further comprising means in thedistributing means for creating turbulence therein and transforming thesuspension flow into a turbulent flow before it is passed into theheadbox through the apertured front wall.

9. The improved apparatus of claim 5, in which the smallest dimension ofthe outlet channel perpendicular to the flow direction does not exceedtwice the mean particle length of the fibrous particles.

10. In an apparatus for the manufacture of a continuous material web offibrous particles the combination of:

a series of units for dividing a suspension flow of the particles in aliquid into a plurality of turbulent flows with successively reducedscale and increased intensity, each unit comprising a tubular chamberwith an inlet at one end thereof; a striking surface at the oppositeend; a plurality of smaller diameter peripheral outlets around thestriking surface; and inside the tubular chamber between the inlet andthe outlets a constricted portion to increase the velocity of theturbulent flow before it impinges onto the striking surface;

a headbox comprising a chamber having at least one transversal row ofinlets for introducing the turbulent flow from the series of units intothe chamber over the entire width thereof; a striking surface fordeflecting the incoming flow; an enlarged portion for decaying the flowto produce a flow of a threedimensional network structure of theparticles with entrained liquid; and an outlet channel, the chamberhaving such dimensions as to prevent any body having a diameterexceeding the threefold mean particle length from passing therethrough;

means connecting the outlets of the units to the inlet of one next unit;and

means connecting each outlet of the last units to one inlet of thetransversal row of inlets.

11. The apparatus of claim 10 in which the headbox chamber comprises twosuperposed rows of inlets and a baffle extending therebetween from thewall of the chamber opposite the outlet channel to provide two oppositestriking surfaces for particles passing through the inlets, one strikingsurface for each row of inlets.

12. The apparatus of claim 10, in which the inlets of one row aredisplaced in relation to the inlets in the other row.

13. The apparatus of claim 10, wherein the total cross-sectional area ofthe outlets of each unit is equal to the cross-sectional area of theinlet of the same unit.

1. AN IMPROVED METHOD OF MANUFACTUREING A CONTINUOUS MATERIAL WEB OFELONGATED FIBROUS PARTICLES INCLUDING PREPARING A SUSPENSION OF THEPARTICLES IN A LIQUID WITH THE CONCENTRATION OF THE FIBROUS PARTICLES INSUSPENSION BEING ABOUT 1.5 PERCENT TO 6.0 PERCENT OF THE TOTAL WEIGHT OFTHE LIQUID PLUS THE FIBROUS PARTICLES; THEN FORCING THE SUSPENSIONTHROUGH A SERIES OF CONSTRICTIONS AND DEFLECTING THE FLOW OF THESUSPENSION AFTER EACH CONSTRICTION TO CREATE A TURBULENT FLOW ENSURINGUNIFORM DISTRIBUTION OVER THE WEB; AND FINALLY DECAYING THE TURBULENTFLOW BEFORE DEPOSITION OF THE MATERIAL AND COMPRISING THE STEPS OF:GRADUALLY DECREASING THE SCALE AND INCREASING THE INTENSITY OF THETURBULENT FLOW IN THE SERIES OF CONSTRICTIONS BY DISTRIBUTING THE FLOWFROM ONE CONSTRICTION OVER A PLURALITY OF SUBSEQUENT CONSTRICTIONS OFSMALLER DIAMETER AND SAID ONE CONSTRICTION, THE SMALLEST CO*STRICTIONSOF SAID SERIES HAVING A DIAMETER NOT EXCEEDING THREE TIMES A MEANPARTICLE LENGTH OF SAID FIBROUS PARTICLES; AND ELIMINATING ALLDISRUPTING FORCES AFTER THE FLOW HAS PASSED THROUGH THE SMALLESTCONSTRICTIONS TO TRANSFORM THE TURBULENT FLOW INTO A FLOW OF ACONSOLIDATED THREE-DIMENSIONAL NETWORK STRUCTURE OF FIBROUS PARTICLESWITH ENTRAINED LIQUID BEFORE DEPOSITION THEREOF.
 1. An improved methodof manufacturing a continuous material web of elongated fibrousparticles including preparing a suspension of the particles in a liquidwith the concentration of the fibrous particles in suspension beiNgabout 1.5 percent to 6.0 percent of the total weight of the liquid plusthe fibrous particles; then forcing the suspension through a series ofconstrictions and deflecting the flow of the suspension after eachconstriction to create a turbulent flow ensuring uniform distributionover the web; and finally decaying the turbulent flow before depositionof the material and comprising the steps of: gradually decreasing thescale and increasing the intensity of the turbulent flow in the seriesof constrictions by distributing the flow from one constriction over aplurality of subsequent constrictions of smaller diameter and said oneconstriction, the smallest constrictions of said series having adiameter not exceeding three times a mean particle length of saidfibrous particles; and eliminating all disrupting forces after the flowhas passed through the smallest constrictions to transform the turbulentflow into a flow of a consolidated three-dimensional network structureof fibrous particles with entrained liquid before deposition thereof. 2.The improved method of claim 1, wherein the flow rate of the networkstructure is successively decreased to the speed of the deposited web.3. The improved method of claim 1, comprising preparing the suspensionwith a fibre concentration of 4-5 percent by weight.
 4. The improvedmethod of claim 1 wherein said smallest constrictions have a diameternot exceeding about 15 mm.
 5. An improved apparatus for the manufactureof a continuous material web of fibrous particles having a length ofabout 1 to 5 mm and of the type having a base; a support on the base; aheadbox attached to the support and comprising a transversal aperturedfront wall and an inside chamber for decaying a turbulent flow beforedeposition thereof; means connected to the headbox for distributing asuspension of the particles in a liquid over the entire width of theheadbox; means connected to the distributing means for preparing thesuspension; and means for transporting the suspension to thedistributing means, the improvement comprising: inside the headbox atleast one other aperture wall in close proximity to the apertured frontwall providing points of impingement for the turbulent flow forcedthrough apertures to deflect the flow, the apertures in said other wallbeing smaller than and displaced with respect to the apertures of saidfront wall so that each point of impingement is surrounded by aplurality of apertures of less diameter than the next preceding apertureso as to successively decrease the scale and increase the intensity ofthe turbulent flow and the apertures of the last of said walls having adiameter not exceeding the threefold mean particle length; and bafflesbehind the last apertured wall defining a chamber wherein the turbulentflow is deflected before it is passed into an outlet channel.
 6. Theimproved apparatus of claim 5, wherein the total area of the aperturesin the front wall is equal to the total area of the smaller diameterapertures in said other apertured wall.
 7. The improved apparatus ofclaim 5, in which the cross-sectional area of the outlet channel isincreased in the flow direction to decrease the velocity of the networkstructure flow to the speed of the deposited web.
 8. The improvedapparatus of claim 5, further comprising means in the distributing meansfor creating turbulence therein and transforming the suspension flowinto a turbulent flow before it is passed into the headbox through theapertured front wall.
 9. The improved apparatus of claim 5, in which thesmallest dimension of the outlet channel perpendicular to the flowdirection does not exceed twice the mean particle length of the fibrousparticles.
 10. In an apparatus for the manufacture of a continuousmaterial web of fibrous particles the combination of: a series of unitsfor dividing a suspension flow of the particles in a liquid into aplurality of turbulent flows with succeSsively reduced scale andincreased intensity, each unit comprising a tubular chamber with aninlet at one end thereof; a striking surface at the opposite end; aplurality of smaller diameter peripheral outlets around the strikingsurface; and inside the tubular chamber between the inlet and theoutlets a constricted portion to increase the velocity of the turbulentflow before it impinges onto the striking surface; a headbox comprisinga chamber having at least one transversal row of inlets for introducingthe turbulent flow from the series of units into the chamber over theentire width thereof; a striking surface for deflecting the incomingflow; an enlarged portion for decaying the flow to produce a flow of athree-dimensional network structure of the particles with entrainedliquid; and an outlet channel, the chamber having such dimensions as toprevent any body having a diameter exceeding the threefold mean particlelength from passing therethrough; means connecting the outlets of theunits to the inlet of one next unit; and means connecting each outlet ofthe last units to one inlet of the transversal row of inlets.
 11. Theapparatus of claim 10 in which the headbox chamber comprises twosuperposed rows of inlets and a baffle extending therebetween from thewall of the chamber opposite the outlet channel to provide two oppositestriking surfaces for particles passing through the inlets, one strikingsurface for each row of inlets.
 12. The apparatus of claim 10, in whichthe inlets of one row are displaced in relation to the inlets in theother row.